The present invention relates to a process for producing polyurethane foam moldings where the density of the molding is at most 450 g/L, by mixing the following to give a reaction mixture: a) organic polyisocyanates with b) polyesterols, c) blowing agents, d) cell-opening additives selected from the group consisting of homo- or copolymers based on ethylhexyl acrylate, on polybutadiene, on polyisobutene, and on diorganosilicones, or a mixture of two or more of said cell-opening additives, e) silicone-based cell stabilizers and optionally f) chain extenders and/or crosslinking agents, g) catalysts, and h) other auxiliaries and/or additives, and charging the materials to a mold, and permitting them to complete a reaction to give a polyurethane foam molding. The present invention further relates to polyurethane moldings obtainable by this process, and to the use of said moldings as shoe sole, steering wheel, seat, or armrest.
Within recent years, there has been a trend toward lower-weight shoe soles. However, in the case of polyurethane shoe soles the density reduction causes problems with the dimensional stability of the moldings. This means that the entire sole becomes smaller, or else that the surface quality of the shoe soles suffers because there are locations affected by shrinkage.
The literature discusses various ways of improving the dimensional stability of the polyurethanes. By way of example, DE 2402734 describes the production of integral polyurethane foams in which a prepolymer based on polyesterol is mixed with a polyol component based on polyetherols. A disadvantage of the resultant polyurethane systems is that, because the polyesterols and polyetherols are incompatible, mechanical properties are adversely affected and it is impossible to prevent shrinkage of the integral polyurethane foams.
The literature also discusses the use of graft polyols or of polymer polyols. By way of example, EP 1 042 384 describes the production of low-density dimensionally stable shoe soles based on polyetherol via use of large amounts of polyether graft polyols. A disadvantage of this process is that mechanical properties are markedly poorer than those of shoe soles based on polyesterol. Furthermore, the high content of polymer polyetherols has a disadvantageous effect on the viscosity of the polyol component.
EP 1 790 675 and EP 1 756 187 describe the addition of polymer polyols based on polyesterols to a polyester polyurethane. These systems are markedly more difficult to process because of the relatively high viscosity resulting from the large amounts of polyester polymer polyol. EP 1 790 675 and EP 1 756 187 also disclose the addition of polyetherol-based polymer polyols to a polyesterol polyurethane system. The comparative example of the specifications reveals that the use of polymer polyetherols leads to an integral foam with inadequate surface and coarse cell structure.
It was an object of the present invention to provide a process which can produce polyurethane foam moldings with density below 450 g/L, and in particular which can produce an integral polyurethane foam which has excellent surface quality and does not shrink during or immediately after its production.